1. Field of the Invention
The present invention relates to a MOS (Metal Oxide Semiconductor) integrated circuit (hereinafter referred to as "MOS-IC") of a multilevel power supply system, wherein a source potential is divided into a plurality of levels to operate an independent circuit at each level.
2. Description of Related Art
Generally, a conventional MOS-IC is supplied with a source potential having a single level from Outside and performs full-swing operation on the basis of the source potential. In some of MOS-ICs, the source potential having a single level applied from outside is stepped down internally to reduce power consumption.
FIG. 1 is a circuit diagram showing an example of a power supply circuit in a conventional MOS-IC, and an inverter is shown in this case. The inverter has a structure, wherein the drain of a P-channel transistor (hereinafter referred to as "P-ch Tr") 2801 is connected to the drain of an N-channel transistor (hereinafter referred to as "N-ch Tr") 2802, source voltage Vcc is applied to the source of the P-ch Tr 2801, and the source of the N-ch Tr 2802 is grounded. To the output node of the inverter, which is the connecting point of the drains, a load capacitor 2803 the one terminal of which is set at ground potential Gnd is connected. In this inverter, either of the transistors turns on depending on input signals applied to the gates of both transistors, and the source potential Vcc or the ground potential Gnd is outputted. At this time, the load capacitor 2803 is charged or discharged.
As shown in this example, in the power supply system of the conventional MOS-IC, when the output of the inverter rises and falls, the load capacitor 2803 connected to the output node thereof is charged or discharged. Power is consumed by this single pair of rising and falling operations.
Since the power supply circuit of the conventional MOS-IC is structured as described above, electrical charges flow to the ground side when discharge occurs at the gate, thereby causing the problem of consuming significant power.